Some applications require the handling, and, more particularly, the pumping of cryogenic liquids. For example, heavy machines like locomotives or large mining trucks may have engines that use more than one fuel. The engine may be a dual fuel engine system, in which a gaseous fuel, such as compressed natural gas, is injected into a cylinder at high pressure while combustion in the cylinder from a diesel pilot is already underway. With such engines, the gaseous fuel is stored in a liquid state at a low pressure, such as atmospheric pressure, and at low, cryogenic temperatures in a storage tank in order to achieve a higher storage density. However, the use of such a cryogenic fuel requires the use of specialized equipment, including a cryogenic tank for storing the liquefied natural gas (“LNG”) fuel and a cryogenic pump for withdrawing and pressurizing the liquefied natural gas fuel.
Dual fuel engines (natural gas+diesel) require high injection pressure of the natural gas to achieve significant greenhouse gas reduction benefits. The most efficient way to generate high-pressure natural gas is to pump it to pressure in liquid form and then heat it. The cryogenic temperatures associated with LNG require highly specialized pump design features. Current designs of high-pressure LNG pumps utilize reciprocating piston pumps which are driven by a crank-slider mechanism. The crank slider mechanisms are large, heavy, and challenging to package on a vehicle.
Wobble shaft pumps have been used in hydraulic and fuel system applications for many years and served their respective industries well. Such a pump is disclosed, for example, in PCT Publication WO 94/03708, which utilized five roller bearing assemblies in support of a rotating shaft. Typical wobble plate pump designs use ball, cylindrical, and tapered roller bearings to carry the axial and radial loads produced by the pumping action. Over the years as pump displacements and pressures have increased so have the loads placed on the wobble plate bearings. Additionally the desire for greater power density has limited the envelope in which the pump and its associated bearings must fit. Ball bearings, while adequate for limited life or light load applications, simply do not have the load capacity to provide the required life of heavy-duty machines and engines, where 10,000-20,000 hour life is the norm. Cylindrical thrust bearings have good capacity, but in thrust applications, the rollers skid across the upper and lower raceways, eventually leading to bearing failure. Tapered roller bearings have good life, but due to their design tend to result in overly large dimensions and are therefore difficult to package. To date, however, commercially available pumps do not support the high loads while providing desirable bearing life in a relatively compact package.